In the manufacture of thin film resistor substrates for thermal ink jet printheads, it is known to provide heater resistors on a common substrate, such as silicon, and employ these resistors to transfer thermal energy to corresponding adjacent ink reservoirs during a thermal ink jet printing operation. This thermal energy will cause the ink in the reservoirs to be heated to boiling and thereby be ejected through an orifice in an adjacent nozzle plate from which it is directed onto a print medium. During such operation, there heater resistors are electrically pulsed by current applied thereto via conductive traces formed on top of the silicon substrate and insulated therefrom by an intermediate dielectric layer. The formation of this layer, the formation of the resistive layer for the heater resistors, and the aluminum evaporation or sputtering process for forming electrical patterns of conductive trace material to the heater resistors are all well known in the art and therefore are not described in further detail herein. However, for a further discussion of the varoius processes used in the fabrication of thermal ink jet printheads, reference may be made to the Hewlett Packard Journal, Volume 36, Number 5, May 1985, incorporated herein by reference.
In order to provide electrical connections between external pulse drive circuits and these conductive traces on the thermal ink jet printhead, it has been a common practice to employ so called flexible or "flex" circuits to make removeable pressure contacts to certain conductive terminal pads on the thin film resistor printhead substrate. For this connection it becomes necessary to provide means by which pressure can be applied to the flexible circuit so that the electrical leads therein make good electrical connection with corresponding mating pads on the thin film resistor substrate.
The flexible circuit will typically consist of a photolithographically defined conductive pattern which has been formed by metal evaporation or sputtering and etching processes carried out on a thin flexible insulating substrate member. These electrical contact locations on the flex circuit will be raised slightly in a bump and dimple configuration, and this geometry may be achieved by the use of a "bed of nails" punch structure which match the location of the dimples. This structure is used to punch the electrical contact locations on the flex circuit to a raised location above the surface of the insulating substrate member thereof.
During this latter punch process, it sometimes happens that not all of the raised contact bumps in the flexible circuit are moved the same distance above the insulating substrate surface. For this reason, more force is necessary to make contact with the smaller or lowerheight bumps than those higher bumps more extended from the surface of the flex circuit.
One approach to providing the necessary force to the flex circuit and the necessary pressure contact between the flex circuit and conductive pads on the thin film resistor substrate is to use an elastomeric material, such as rubber, which has been preformed to have a plurality of cones spaced at locations corresonding to the location of the dimples in the flex circuit. In this manner, the tips of these cones can be inserted into the dimples of the flex circuit and urged thereagainst with a force sufficient to bring the conductive bumps on the flex circuit in to good physical and electrical contact with the terminal pads on the thin film resistor substrate.
While the above prior art approach to making electrical contact between the flex circuit and the print-head substrate has proven satisfactory for certain types of interconnect patterns with few interconnect members, it has not been entirely satisfactory for connecting larger numbers of conductive traces to larger numbers of conductive bumps or pads on the flex circuit. This fact has been a result of the nature of the nonlinear deflection of the above elastomeric cones. This nonlinear deflection of the elastomeric cones is seen as a nonlinear variation in cone volumetric compression, V.sub.c, as a function of the distance, D, that the tip of the cone is moved during an interconnect operation. Thus, this nonlinear characteristic tends to increase the amount of force which must be applied to the flex circuit in order to insure that all the bumps on the flex circuit make good electrical contact with the conductive traces or terminal pads on the printhead substrate. In some cases this required force is sufficiently large to fracture the substrate or do other structural damage thereto. This non-linear deflection characteristic of the prior art is described in more detail below with reference to the prior art FIGS. 1A and 1B.